1. Field of the Invention
This invention relates to the field of current measurement, and relates to a current measurement device equipped with an improved current sensor with variable magnetic excitation, to measure a direct or alternating current. The current measurement device of the invention comprises, on a same support, excitation and detection windings, as well as at least one winding that the current to be measured is designed to pass through. The invention applies to the field of microelectronics and micro-technologies and provides for the production of a micro current sensor, that is robust as concerns voltage surges, and which has improved performances especially as concerns the bandwidth, the dissipated power and immunity to environmental electromagnetic interferences.
2. Description of the Related Art
There are many different techniques for measuring an electrical current. In particular, resistance isolation technologies may be used, for which the measurement of the current is indirect and carried out by the detection of a magnetic field created by this current. The measurement of a high current may be especially made using a shunt or a current transformer, whose role is to adapt the high current to be measured to a component. Another method that is traditionally used is the ampere metric magnetic cell clip, for example the “Hall effect” type, or the “fluxgate” type, or the “Rogowski” type. The type of sensor used to measure electrical current will be chosen according to the specific needs of the application, such as:                the measurement range, the peak or efficient value of the current maximal to be measured,        the accuracy and/or resolution desired,        the bandwidth, the maximum frequency, the measurement or not of the direct current.        
A shunt can be assimilated to an accurate low value resistor. The current to be measured passes through this resistor and causes a voltage drop to its terminals, which is transmitted to a signal processing device such as for example an analogue/digital converter. The precision resistor generally comprises 4 terminals to avoid unwanted parasite effects of the connections. A shunt may comprise two terminals to connect the current circuit, and two junction terminals to measure the voltage drop. The shunt allows direct current measurements to be made, but has in particular a bandwidth limited to several kHz typically, and introduces interference.
A current transformer generally comprises a “primary winding” and a “secondary winding” around a magnetic circuit. A current that is to be measured runs through the primary which has a given number N1 of spires of high cross section, generally a single spire, whereas the secondary has a number N2 of spires of low cross section. Such a type of sensor stands up well to voltage surges or short-circuits and has a bandwidth that may extend for example from 50 Hz to 50 kHz. The transformer sensors however have the disadvantage of not permitting a direct current to be measured.
A current clip device is generally formed by a magnetic circuit that may be open, a magnetic sensor, for example of the Hall effect type and a counter reaction winding. The clip surrounds a conductor or a set of conductors through which, a current that is to be measured, passes. With such a device, the measurement is made entirely without any contact and without physical connection to the conductor through which the current to be measured passes.
As concerns the current clip devices, there are specific current clips for “Rogowski” type sensors that are not equipped with a magnetic circuit. The document: “PCB Rogowski coil designs and performances for novel protective relaying” Kojovic, Power Engineering Society General Meeting, IEEE Volume 2, 13-17 Jul. 2003, presents such a type of sensor. One or several coils are coupled to the conductor though which passes the current to be measured, and enables a voltage to be induced. With such a type of sensor, the measurement offset and gain depend on many physical and geometrical parameters such as the size of the air gap, the relative position of the sensor in the air gap, the centring of the conductor in the clip, the permeability of the magnetic circuit, the proximity of any possible magnetic objects, the amplitude and the direction of external interfering magnetic fields.
The fluxgate is a very sensitive magnetic field sensor, whose bandwidth extends from continuous to several dozen kHz. The sensor fluxgate comprises, around a magnetic circuit with very high magnetic permeability, an excitation coil and a detection coil. The excitation coil saturates the magnetic circuit alternatively under a positive and negative field. The detection coil is sensitive to an imbalance between the positive and negative field. This imbalance is caused by an external source, for example the magnetic earth field in the case of a field sensor. In the case of a current clip with fluxgate sensor, the imbalance is caused by the current to be measured flowing in a central conductor. The conductor in which flows the current to be measured and the sensor form two elements that are independent or that are not in contact. In one such device, the measurement performances depend on the control of the relative position of the sensor with respect to the conductor. Furthermore, the distance between the conductor of the current to be measured is around one millimeter and is difficult to reduce to a very low value.
There is consequently the problem of finding a new current sensor device, which does not have the disadvantages mentioned above.